1. Field of the Invention
The invention relates to a projection objective for imaging a pattern of a mask arranged in an object surface of the projection objective into an image field arranged in the image surface of the projection objective with a demagnifying imaging scale, and to a microlithography projection exposure apparatus having such a projection objective.
2. Description of the Related Prior Art
Photolithographic projection objectives with a demagnifying imaging scale (reduction objectives) have been used for several decades for the photolithographic fabrication of semiconductor components and other finely patterned devices. They serve for projecting the pattern of a mask, e.g. of a photomask or of a reticle, onto an article coated with a light-sensitive layer with very high resolution on a demagnifying scale.
Conventional projection systems are designed to image a planar mask onto a planar image field. Accordingly, measures for correcting the image field curvature (Petzval correction) are provided in the projection objectives. The article “New lenses for microlithography” by E. Glatzel in: SPIE Vol. 237 (1980), pp. 310-320, describes known measures for leveling the image field.
The U.S. Pat. No. 5,052,763 describes a catadioptric projection objective with intermediate image, wherein the image of the object field (intermediate image) generated by a first, catadioptric subsystem is imaged into the image plane with the aid of a second, refractive subsystem. In order to be able to image a planar object into a planar image surface, the Petzval sum of the system is obtained by compensation of the image field curvature generated by the first subsystem by means of the second subsystem, a curved intermediate image surface being generated.
The U.S. Pat. No. 4,812,028 describes a microlithography projection objective having aplanatic refracting surfaces, non-aplanatic refracting surfaces and reflective surfaces. The Petzval sums of the aplanatic refracting surfaces and of the remaining surfaces are corrected independently of one another.
For projection lithography onto curved substrates, the U.S. Pat. No. 6,461,908 B1 proposes using a curved mask whose form is identical to the form of the curved substrate. The curved mask is produced in a contact method. Curvature-conforming imaging of the curved mask onto the curved substrate requires projection objectives with a substantial correction of the image field curvature.
The U.S. Pat. No. 5,527,139 discloses a purely reflective reduction objective for extreme ultraviolet radiation (EUV), wherein the object surface and/or the image surface are curved concavely with respect to the projection objective.
The patent application US 2003/0133087 A1 describes a method by which imaging errors that may result on account of the gravitational-force-dictated curvature of a reticle are intended to be prevented. This involves taking account of the fact that a gravitational-force-dictated mask bending can lead to a distortion of the pattern situated on the mask (pattern stretching), so that errors similar to a distortion error result in the image of the pattern. In order to correct these errors, an optical correction element designed for a distortion correction is introduced between the object plane and the projection objective.